net/ipv4/ipv6: Replace one-element arraya with flexible-array members

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Use an anonymous union with a couple of anonymous structs in order to
keep userspace unchanged and refactor the related code accordingly:

$ pahole -C group_filter net/ipv4/ip_sockglue.o
struct group_filter {
	union {
		struct {
			__u32      gf_interface_aux;     /*     0     4 */

			/* XXX 4 bytes hole, try to pack */

			struct __kernel_sockaddr_storage gf_group_aux; /*     8   128 */
			/* --- cacheline 2 boundary (128 bytes) was 8 bytes ago --- */
			__u32      gf_fmode_aux;         /*   136     4 */
			__u32      gf_numsrc_aux;        /*   140     4 */
			struct __kernel_sockaddr_storage gf_slist[1]; /*   144   128 */
		};                                       /*     0   272 */
		struct {
			__u32      gf_interface;         /*     0     4 */

			/* XXX 4 bytes hole, try to pack */

			struct __kernel_sockaddr_storage gf_group; /*     8   128 */
			/* --- cacheline 2 boundary (128 bytes) was 8 bytes ago --- */
			__u32      gf_fmode;             /*   136     4 */
			__u32      gf_numsrc;            /*   140     4 */
			struct __kernel_sockaddr_storage gf_slist_flex[0]; /*   144     0 */
		};                                       /*     0   144 */
	};                                               /*     0   272 */

	/* size: 272, cachelines: 5, members: 1 */
	/* last cacheline: 16 bytes */
};

$ pahole -C compat_group_filter net/ipv4/ip_sockglue.o
struct compat_group_filter {
	union {
		struct {
			__u32      gf_interface_aux;     /*     0     4 */
			struct __kernel_sockaddr_storage gf_group_aux __attribute__((__aligned__(4))); /*     4   128 */
			/* --- cacheline 2 boundary (128 bytes) was 4 bytes ago --- */
			__u32      gf_fmode_aux;         /*   132     4 */
			__u32      gf_numsrc_aux;        /*   136     4 */
			struct __kernel_sockaddr_storage gf_slist[1] __attribute__((__aligned__(4))); /*   140   128 */
		} __attribute__((__packed__)) __attribute__((__aligned__(4)));                     /*     0   268 */
		struct {
			__u32      gf_interface;         /*     0     4 */
			struct __kernel_sockaddr_storage gf_group __attribute__((__aligned__(4))); /*     4   128 */
			/* --- cacheline 2 boundary (128 bytes) was 4 bytes ago --- */
			__u32      gf_fmode;             /*   132     4 */
			__u32      gf_numsrc;            /*   136     4 */
			struct __kernel_sockaddr_storage gf_slist_flex[0] __attribute__((__aligned__(4))); /*   140     0 */
		} __attribute__((__packed__)) __attribute__((__aligned__(4)));                     /*     0   140 */
	} __attribute__((__aligned__(1)));               /*     0   268 */

	/* size: 268, cachelines: 5, members: 1 */
	/* forced alignments: 1 */
	/* last cacheline: 12 bytes */
} __attribute__((__packed__));

This helps with the ongoing efforts to globally enable -Warray-bounds
and get us closer to being able to tighten the FORTIFY_SOURCE routines
on memcpy().

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 10 insertions, 9 deletions

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index 468969c75708..b297bb28556e 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -790,7 +790,8 @@ static int ip_set_mcast_msfilter(struct sock *sk, sockptr_t optval, int optlen)
 		goto out_free_gsf;
 
 	err = set_mcast_msfilter(sk, gsf->gf_interface, gsf->gf_numsrc,
-				 gsf->gf_fmode, &gsf->gf_group, gsf->gf_slist);
+				 gsf->gf_fmode, &gsf->gf_group,
+				 gsf->gf_slist_flex);
 out_free_gsf:
 	kfree(gsf);
 	return err;
@@ -799,7 +800,7 @@ out_free_gsf:
 static int compat_ip_set_mcast_msfilter(struct sock *sk, sockptr_t optval,
 		int optlen)
 {
-	const int size0 = offsetof(struct compat_group_filter, gf_slist);
+	const int size0 = offsetof(struct compat_group_filter, gf_slist_flex);
 	struct compat_group_filter *gf32;
 	unsigned int n;
 	void *p;
@@ -813,7 +814,7 @@ static int compat_ip_set_mcast_msfilter(struct sock *sk, sockptr_t optval,
 	p = kmalloc(optlen + 4, GFP_KERNEL);
 	if (!p)
 		return -ENOMEM;
-	gf32 = p + 4; /* we want ->gf_group and ->gf_slist aligned */
+	gf32 = p + 4; /* we want ->gf_group and ->gf_slist_flex aligned */
 
 	err = -EFAULT;
 	if (copy_from_sockptr(gf32, optval, optlen))
@@ -826,7 +827,7 @@ static int compat_ip_set_mcast_msfilter(struct sock *sk, sockptr_t optval,
 		goto out_free_gsf;
 
 	err = -EINVAL;
-	if (offsetof(struct compat_group_filter, gf_slist[n]) > optlen)
+	if (offsetof(struct compat_group_filter, gf_slist_flex[n]) > optlen)
 		goto out_free_gsf;
 
 	/* numsrc >= (4G-140)/128 overflow in 32 bits */
@@ -834,7 +835,7 @@ static int compat_ip_set_mcast_msfilter(struct sock *sk, sockptr_t optval,
 	if (n > sock_net(sk)->ipv4.sysctl_igmp_max_msf)
 		goto out_free_gsf;
 	err = set_mcast_msfilter(sk, gf32->gf_interface, n, gf32->gf_fmode,
-				 &gf32->gf_group, gf32->gf_slist);
+				 &gf32->gf_group, gf32->gf_slist_flex);
 out_free_gsf:
 	kfree(p);
 	return err;
@@ -1455,7 +1456,7 @@ static bool getsockopt_needs_rtnl(int optname)
 static int ip_get_mcast_msfilter(struct sock *sk, void __user *optval,
 		int __user *optlen, int len)
 {
-	const int size0 = offsetof(struct group_filter, gf_slist);
+	const int size0 = offsetof(struct group_filter, gf_slist_flex);
 	struct group_filter __user *p = optval;
 	struct group_filter gsf;
 	int num;
@@ -1467,7 +1468,7 @@ static int ip_get_mcast_msfilter(struct sock *sk, void __user *optval,
 		return -EFAULT;
 
 	num = gsf.gf_numsrc;
-	err = ip_mc_gsfget(sk, &gsf, p->gf_slist);
+	err = ip_mc_gsfget(sk, &gsf, p->gf_slist_flex);
 	if (err)
 		return err;
 	if (gsf.gf_numsrc < num)
@@ -1481,7 +1482,7 @@ static int ip_get_mcast_msfilter(struct sock *sk, void __user *optval,
 static int compat_ip_get_mcast_msfilter(struct sock *sk, void __user *optval,
 		int __user *optlen, int len)
 {
-	const int size0 = offsetof(struct compat_group_filter, gf_slist);
+	const int size0 = offsetof(struct compat_group_filter, gf_slist_flex);
 	struct compat_group_filter __user *p = optval;
 	struct compat_group_filter gf32;
 	struct group_filter gf;
@@ -1498,7 +1499,7 @@ static int compat_ip_get_mcast_msfilter(struct sock *sk, void __user *optval,
 	num = gf.gf_numsrc = gf32.gf_numsrc;
 	gf.gf_group = gf32.gf_group;
 
-	err = ip_mc_gsfget(sk, &gf, p->gf_slist);
+	err = ip_mc_gsfget(sk, &gf, p->gf_slist_flex);
 	if (err)
 		return err;
 	if (gf.gf_numsrc < num)